This is from 2008 but it gives a good overview of Federal Law Enforcement officers for basically just about every agency including ones you would never think had Federal Law Enforcement officers as part of their organization. For instance, the National Institute of Standards & Technology had 28 armed officers as part of its organization.
Also keep in mind that some of these agencies would have taken a major hit during the TDM - i.e. the Secret Service between the President's plane going down and the White House getting nuked - basically what would be left would be the agents who were in field offices or who were off duty and far enough away to survive the DC nukes
https://www.bjs.gov/content/pub/pdf/fleo08.pdf